Production of coated roadstone

ABSTRACT

An apparatus for producing coated roadstone in which a cold aggregate material having a predetermined moisture content and a bituminous binder are charged into a mixer barrel open at both ends while the latter is disposed with its axis inclined to the horizontal. The barrel is pivoted into a position in which its axis is horizontal and rotated without heating so as to produce a cold mix. Mixing continues while hot air is passed through the barrel. When mixing is complete, the barrel is returned to its inclined position and rotated in reverse direction to discharge the mix.

PRODUCTION OF COATED ROADSTONE 3/1954 Comstock 23/90 2,736,288 2/1956 Cla et a1 118/19 175] Inventor: F Laycock, whashton, 2,860,598 11/1958 Loe sche 118/418 Rwhmond, England 2,877,159 3/1959 Lachman et al. 167/82 3,213,827 10/1965 .lenkin 118/495 [73] Assgnee' glgg za sgf yfig 5 53222 3,317,307 5/1967 Wise et al. 71/64 England g 3,494,327 2/1970 Pearson 118/4111 [22] Filed: Apr. 14, 1972 Primary ExaminerMervin Stein Assistant Examiner-Leo Millstein [21] Appl' 243944 Attorney, Agent, or Firm-Markva & Smith 301 Foreign Application Priority Data [57] ABSTRACT Apr1 15, 1971 Great Br1ta1n 9515/71 A pp atu fo produc g co te roa s o i which a cold aggregate material having a predetermined '8 118/418 l moisture content and a bituminous binder are charged [58] Fie'ld 417 418 into a mixer barrel open at both ends while the latter 259/3 14 18 is disposed with its axis inclined to the horizontal. The barrel is pivoted into a position in which its axis is hor- [56] References Cited izontal and rotated without heating so as to produce a cold mix. Mixing continues while hot air is passed UNITED STATES PATENTS through the barrel, When mixing is complete, the barl,ll6,94l 11/1914 Brackenbury 259/158 X rel is returned to its inclined position and rotated in reverse direction to discharge the mix.

e ler e a. 2,572,743 /1951 Mills 91/122 (Ilaims, 9 Drawing Figures 1 l/ \IJ fifiZfi fj,/' II 120 ,79 23 122 f Z I2 13 s 6 HTF l} a 1 1' 1 I r- I I 17 a l ML o LR r-l6 T''!1 '1 j =7 HA I 11 I 1 7 17 I I I I V I 3 I 5 3'1 1 1o 32 317 May 28, 1974 PATENTEDMAY 28 1914 3 8 l 2 821 SHEET u or 4 PRODUCTION OF COATED ROADSTONE BACKGROUND OF THE INVENTION This invention relates to an apparatus for the production of coated roadstone. I

In the customary production process, the starting aggregate material, generally having natural water content of about three to five percent by weight, is first dried by heating it in a dryer and then the dried aggregate material is graded, proportioned and mixed hot in a mixer with a suitable quantity of a liquid bituminous binder, for example a bitumen emulsion or liquid bitumen, to produce a hot coated roadstone ready for use. The pre-drying of the aggregate material, however, releases dust and fine constitutents which were previously held by the damp material, with the result that these are carried off in the current of heated air and water vapour from the dryer. Expensive dust collection equipment, such as cyclone separators, have to be provided not only for recovering these entrained constituents and returning them to the aggregate material where they are needed for filling voids between coarser constituents but also for reducing dust emission to a level complying with official regulations concerning industrial emissions.

Processes have been proposed in the past in which a moist aggregate material is fed to a mixer into which a binder is introduced. In some processes the aggregate material is heated to remove the moisture and in general heat is applied to the aggregate material either as soon as the latter comes into contact with the binder or even before it comes into contact with the binder. In other processes, an additive, for example a chromium or barium salt, is employed to displace the moisture and permit the binder to adhere to the aggregate material. By using such an additive the temperature at which the coating process is carried out can be kept relatively low.

It has been found that a process which is virtually dustlcss is obtained provided the binder and aggregate are subjected to a thorough mixing operation in the cold before the mix is subjected to heating. Because the aggregate material and binder have been mixed to form an intimate mixture by the time heating takes place, the dusty constituents of the aggregate material are almost entirely retained in the mix and cannot be carried away with the heated air and water vapour. In addition, it has been discovered that there are advantages in ensuring that the aggregate material has a moisture content of between one half and percent by weight since thereby the coating of the aggregate material with the binder is promoted. It has also been found that the properties of the product can be considerably improved and maintained constant if the aggregate material and binder is mixed in batches.

The object of the present invention is therefore to provide an apparatus for the batch mixing of aggregate material and binder according to the process outlined above.

SUMMARY OF THE INVENTION Accordingly, the present invention provides an apparatus for producing coated roadstone comprising charging into a mixer a cold aggregate material having a predetermined moisture content and a bituminous binder, mixing the binder and aggregate material together firstly in the absence of heat so as to form a cold mix in which solid particles of binder are interspersed among the moist aggregate material, and secondly in the presence of heat so that the moisture evaporates from the aggregate material and the binder liquefies and coats the aggregate material, discharging the coated product from the mixer, and cooling the mixer. The process carried out in accordance with the present invention renders unnecessary any additive to displace the moisture adhering to the aggregate material, this moisture being removed by evaporation.

The binder may be added to the aggregate material while the latter is in a weigh hopper preparatory to being discharged into the mixer or on a conveyor belt leading to the weigh hopper but the binder is preferably charged directly into the mixer at about the same time as the aggregate material. The binder may be added in granular form but it is preferably injected in a molten condition and solidifies in contact with the moist and cold aggregate material.

Although other heating and cooling systems may be employed, the heat is preferably supplied during the second mixing operation by passing a current of hot air through the mixer while at the end of the process the mixer is cooled by passing through it a current of cooling air.

Although the complete process is preferably carried out in one and the same mixer, according to a modification of the invention, the mix may be discharged before being heated and may be conveyed in a batch to a storing area. When it is required for use it is charged into the same mixer or another mixer possibly at a remote site and is therein subjected to mixing in the presence of heat.

A preferred apparatus for carrying out the methods described above comprises a hopper for moist aggregate material, an outlet for a bituminous binder, a mixer, an air heater, a first air supply duct, positioned to communicate with the mixer in 'a mixing position, first valve means for directing along or diverting from said duct a current of air from said heater, an air moving device, a second air supply duct positioned to communicate with the mixer in a discharge position, second valve means for directing along or diverting from said second duct a current of air from said air moving device, and control means for initiating a first phase in which the mixer is positioned in a charging position to receive aggregate material and binder discharged from the hopper and binder outlet, and thereafter moving the mixer to a mixing position, initiating a second phase in which the mixer is operated to effect mixing of its contents, firstly with said first valve means positioned to divert heated air from the first duct and secondly with said first valve means positioned to direct heated air to the first duct, and initiating a third phase in which the mixer is moved to a discharge position and, after the contents of the mixer have at least partly discharged, the second valve means is positioned to direct air from the air moving device to the second air supply duct.

The control means may be manually operated or may be automatic in operation, and arranged to initiate the various steps at predetermined times depending upon the setting given to the control means Sensing devices responsive to the temperature or other condition of the mix may be employed to initiate the termination of the second phase. The mixer may be a rotary mixing drum or barrel which moves pivotally about a horizontal axis between the various positions. Although the drum may actually move to three different positions, the mix discharging for example from a common charging and discharging opening or mouth, it is preferred for the charging position to be the same as the discharging position. With this arrangement, the drum preferably has separate charging and discharging openings, the former being uppermost and the latter being lowermost when the drum is ready to be charged or discharged. Accordingly, the axis of rotation of the drum may be inclined when the drum is to be charged or discharged and substantially horizontal when the drum is to be rotated during mixing.

A convenient arrangement of the ducting system is achieved if the first air supply duct (i.e., the duct supplied with air heated by the air heater) is aligned during the mixing phase with the discharging opening. Air passed into the mixing drum from the heater may pass out through the charging opening into a third duct which forms the intake of the air moving device which is preferably a centrifugal fan. The second air supply duct conveniently aligns during the charging and discharging phases with the charging opening of the mixer and has opening into it the outlet from the hopper.

The air heater and air moving device are preferably operated continuously while the apparatus is in use. To

conserve fuel, the heater may be turned onto a low flame or pilot flame while the air from the heater is being diverted from the first duct and hence to waste. The air moving device as will be appreciated, not only serves to provide the supply of cooling air but also acts as an extractor fan to assist in drawing the air from the heater through the mixing drum. Except when the drum is being cooled, the air moving device will discharge to atmosphere through a duct or chimney which will also incorporate a valve means which is moved to a closed position when the second valve means is opened. The third duct, which constitutes the intake of the air moving device conveniently incorporates a valve means which is movable between a position in which the air moving device extracts air from the mixer drum and a position in which it draws air through a duct opening to atmosphere. Although, obviously, it is preferred to use air as the heating and cooling medium, the use of other gases for this purpose is intended to fall within the scope of the invention and be contemplated by the references to air.

The various valve means may be dampers which are operated by the control means through the agency of hydraulic or pneumatic rams. The hopper too, may have a discharge door similarly operable.

The apparatus described above is preferably supported on a structural framework, the aggregate material being conveyed along a conveyor belt or similar means to the hopper from which it is fed to the mixer drum. The moisture content of the aggregate material may be tested one or more times before it is charged into the mixer drum and any necessary moisture content adjustments made by spraying water onto the aggregate material, possibly while it is in the hopper or while it is being conveyed to the hopper.

BRIEF DESCRIPTION OF DRAWINGS A specific embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, in which:

' the operation of the plant.

DESCRIPTION OF SPECIFIC EMBODIMENTS Referring to FIGS. 1 to 3, a plant for coating roadstone includes a structural framework 1 which permits a truck T to be driven into a position for receiving hot coated roadstone discharged from a batch mixer, generally designated 2 and supported on a lower platform 30 of the framework. The batch mixer 2 comprises a barrel 3 of double frusto-conical form having a charge mouth4 at its one end. Internally, the barrel 3 is provided with spiral mixing scrolls in its one frusto-conical part 6 and with retaining scrolls or rings in its other frusto-conical part '7 to prevent mix from falling from the discharge mouth 5 of the inclined barrel 3, except when the latter is rotated in a counter-clockwise direction. The barrel 3 is mounted for rotation on its axis on bearings in a frame 8 and is driven through gearing by a hydraulic motor generally designated at 9. The frame 8 carrying the barrel 3 is pivotally mounted between trunnions 10 and is connected at each side to a hydraulic ram 11 operable to tilt the barrel 3 from the horizontal position shown to a position in which its axis of rotation is positioned as shown at an angle of about 45 to the horizontal.

The barrel 3 thus has two positions, namely the position where the axis is horizontal as shown at R and a position where the axis is inclined as shown at Rl. The horizontal position is the position for mixing and the inclined position is the position for charging, discharging and barrel 3 cooling. In each of these positions one or both of the mouths of the barrel are in alignment with the mouths of the ducts of the duct system which will now be described in greater detail.

Mounted on a platform 31 at one end of the framework is an air heater 12. In the preferred embodiment the heater includes an oil or gas burner capable of being set to direct either a full flame or a short flame into a combustion chamber through which air is forced by a blower so that heated air issues into a duct 13. The mouth of the duct 13 is so located that it is aligned with the mouth 5 of the barrel 3 when the axis of rotation of the latter is in its horizontal position. Opening into the duct 13 are an exhaust duct 14 and an intake duct 15. The axes of the ducts I4 and 15 are staggered so that when a damper 113 in the duct 13 is in its closed position the ducts l4 and 15 are isolated from each other, the duct 14 being in connection with the air heater and the duct 15 being in connection with the barrel. A damper 114 is located in the duct 14 and a damper 115 is located in the duct 15. At the other end of the structural framework, an air moving device in the form of an extractor fan 16 is supported on a cantilevered platform 32. The inlet duct 17 of the extractor fan has a mouth which is in alignment with the mouth 4 of the barrel 3 when the axis of rotation of the latter is in its horizontal position. An intake duct 18 opens into the duct 17 and can be closed by a damper 118. Located within the duct 17 is a damper 117 which is positioned between the mouth of the duct 17 and the position at which the duct 18 opens into the duct 17.

The discharge duct 19 from the extractor fan extends upwardly to form a chimney and has branching from it a duct 20, the mouth of which is positioned so as to align with the mouth 4 of the barrel 3 when the axis of rotation of the latter is in its inclined position. A damper 119 is disposed in the duct 19 nearer the mouth of the duct 19 than the position at which the duct 20 opens into it and a damper 120 is disposed within the duct 20.

A conveyor diagrammatically indicated at 21 serves to convey batches of undried aggregate material into a chute 26 leading to a batch check weigh hopper 22 supported on an upper platform 33 of the framework. The hopper 22 has an exit chute 23 which opens into the duct 20 and is fitted with a door or slide valve 122. The door 122 and the various dampers are operable by pneumatic hydraulic rams which operate in a timed sequence under automatic or manual control possibly in response to sensing means which responds to the temperature or another condition of the batch being mixed.

An injection nozzle 23a within the duct 20 is directed towards the mouth of the duct and an injection nozzle 24 is located within and opens towards the mouth of the duct 13. The nozzles 23a and 24 are connected by way of metering and pumping devices to sources of bituminous binder and flux oil respectively.

A nozzle 25 connected to a supply of water is located to direct a spray of water into the chute 26.

The operation of the plant will now be described with reference to FIGS. 4 to 9 in which the door 122, the various dampers. air heater, the barrel 3 and ducting system are illustrated diagrammatically. Currents of hot air from the burner on full flame are indicated by broken lines and currents of warm air from the burner when running at low flame as well as currents of ambient air are represented by solid lines. The operating condition of the burner is indicated by a single flame. In FIGS. 4 to 9 the ducts l3 and 17 are shown for clarity as being rotated through 90 about their axes compared with the barrel and the remaining ducts.

At the commencement of operation, the motors of the compressor, hydraulic power pack, serving the door and dampers, extractor fan, burner blower fan, oil pump, water pump, flux oil pump, binder tank pump and aggregate feed conveyor are started.

The required grades of undried aggregate material drawn from store (which may be storage heaps exposed to the weather) are supplied to feed hoppers delivering the grades onto a conveyor belt through weighing devices such as belt weigh feeders or through volumetric feeders by means of which the proportioning of the different grades forming a batch delivered to the conveyor belt can be controlled. The water content of the grades is measured by moisture content indicators provided, for example, at the feed hoppers, to take the water content into account in weighing out the appropriate quantities of the different grades to ensure that their dry weight will comply with the desired specification of coated roadstone. It has been found that in the hot mixing operation the coating of the aggregate material with the liquefied binder is promoted by the presence of the water which has the effect of improving the flow and speed of the liquefied binder, and also improving its adhesion, on the constituents of the aggregate material.

Consequently, the water content of the batch of aggregate material may be adjusted as necessary or desired to obtain a desired content between one-half and 15 percent, more particularly between 3 to 5 percent or higher by weight, by addition of water to the material. The moisture content of the material is again checked in the weigh hopper 26 and the water spray 25 is set to bring the aggregate to the required moisture content. As soon as the weight indicator begins to register a reading, the correct amount of water is sprayed into the aggregate. The dampers are set to the positions shown in FIG. 4 and the burner is set on a low flame as indicated at 27. The unwanted warm air produced by the burner is exhausted through the duct 14 and the extractor fan 16 draws in air through the duct 18 and discharges it through the outlet duct 19. The barrel 3 is stationary in its inclined position so that when the door 122 is opened under the control of the control system, aggregate A is fed into the barrel 3 and at the same time binder B is injected from the nozzle 23. When the indicator on the weigh hopper 26 registers zero weight the control system closes the door 122 and a signal is given calling the next batch. The mixer or barrel 3 is pivoted by hydraulic ram 11 to the position in which its axis is horizontal, as shown in FIG. 5, and the motor 9 is started so as to rotate the barrel 3 in the clockwise direction. The positions of the dampers remain unchanged so that the mixing of the aggregate and binder takes place in the cold. During the mixing operation shown in FIG. 5, the binder solidifies in contact with the cold aggregate material, and a cold mix of the aggregate material and solid binder particles is formed. The first phase is then completed.

The control system now brings the burner on to full flame at 28 and sets the dampers in the positions as shown in FIG. 6. Hot air is now passed from the air heater through the rotating barrel and is extracted and discharged to atmosphere by the extractor fan.

During the mixing operation shown in FIG. 6, the current of hot gas removes the water from the aggregate material while liquefying the binder so that the material is coated immediately by the binder which holds or fixes the fines of the aggregate material against being picked up or carried off by the current of hot air. If fluxing is required, the burner is set on a low flame and the control system sets the dampers in the positions shown in FIG. 7 so that cold air is drawn through the barrel from the intake duct 15 by the extractor fan. Flux oil is discharged through the nozzle 24.

When mixing, and fluxing if required, have been completed, the second phase is at an end and the mixer 3 is returned to the inclined position as shown in FIG. 8. The motor 9 is cut out and then driven in the reverse direction so that the barrel slows down, comes to a halt and is then rotated in the counter-clockwise direction.

The hot mix is discharged through the mouth 5 into the waiting truck T. During this time, the burner again is set or is maintained on a low flame and the dampers positioned as shown in FIG. 8. When the mix has been discharged, the barrel 3 is cooled while still in its inclined position by opening the damper and closing the damper 119. As shown in FIG. 9, cold air is drawn in through the intake duct 18 and passed into the barrel 3 so as to discharge through the mouth 5. With the third phase completed, the motor is then stopped and the positions of the dampers 119 and 120 reversed to return the system to its FIG. 4; position ready for the start of the next sequence.

Omitting fluxing and assuming a load of 5,000 lbs. possible timings of the operational phases may, for example, be as follows:

First Phase:

Charging of aggregate and binder into the stationary barrel (FIG. 4): O to 9 seconds. Movement of barrel to the horizontal position: 10 to seconds.

Cold mixing of undried aggregate and binder (H6.

5): 16 to 26 seconds.

Second Phase:

Hot mixing of aggregate and binder (FIG. 6): 27 to 87 seconds.

Third Phase:

Barrel moved to inciined position 88 to 93 seconds.

Discharge of product into truck (FIG. 8): 94 to 104 seconds. Cooling of barrel: 105 to l25 seconds. A suitable temperature of the current of hot air is, for example 600C.

The product of the process in accordance with the invention when carried out under laboratory conditions has been found to be superior to the product of conventional processes.

I claim: 1. Apparatus for producing coated roadstone, comprising:

a hopper for moist aggregate material, an outlet for a bituminous binder,

a mixer,

an air heater, a first air supply duct positioned to communicate with the mixer in a mixing position, first valve means for directing along or diverting from said duct a current of air from said heater,

an air moving device, a second air supply duct positioned to communicate with the mixer in a discharge position, second valve means for directing along or diverting from said second duct 21 current of air from said air moving device,

and control means for:

initiating a first phase in which the mixer is positioned in a charging position to receive aggregate material and binder discharged from the hopper and binder outlet, and thereafter moving the mixer to a mixing position,

initiating a second phase in which the mixer is operated to effect mixing of its contents, first with said first valve means positioned to divert heated air from the first duct and secondly with said first valve means positioned to direct heated air to the first duct, and

initiating a third phase in which the mixer is moved to a discharge position and, after the contents of the mixer have at least partly discharged, the second valve means is positioned to direct air from the air moving device to the second air supply duct.

2. Apparatus as claimed in claim 1, comprising a device for testing the moisture content of the aggregate before it has been fed to the hopper and a device for spraying water onto the aggregate.

3. Apparatus as claimed in claim 1, wherein the mixer comprises a rotary mixing drum which is pivotably movable about a horizontal axis between the charging, mixing and discharge positions.

4. Apparatus as claimed in claim 1, wherein the charging position is the same as the discharging position.

5. Apparatus as claimed in claim 1, wherein the mixer has a first or charging opening and a second or discharging opening.

.6. Apparatus as claimed in claim 5, wherein said first air supply duct is alignable with the second opening in the mixer.

7. Apparatus as claimed in claim 5, wherein said second air supply duct is alignable with the first opening in the mixer.

8. Apparatus as claimed in claim 7, wherein the hopper discharges into the second air supply duct.

9. Apparatus as claimed in claim 5, wherein a third duct is aligned with the first opening in the mixer when the mixer is in its mixing position, and said third duct is the intake duct of the air moving device.

10. Apparatus as claimed in claim 1, wherein said control system is operative to bring about a reduction in the heat output of the air heater at or about the same time as the first valve means diverts air from the first duct.

11. Apparatus for the production of coated roadstone, comprising:

a mixer drum which has at least one mouth and is supported for rotation about an axis passing through said at least one mouth,

said drum being pivotally movable between a position in which the axis of rotation is horizontal and at least one position in which the axis of rotation is inclined to the horizontal,

means for charging a moist aggregate and a bituminous binder into the drum when said drum is inclined,

heating means for providing a current of hot air,

a ducting system for causing the hot air to flow through the drum when said drum is positioned with its axis horizontal,

valve means for controlling the flow of hot air through said ducting system, and

control means for initiating a first phase in which charging of the drum takes place with the drum in the inclined position and the drum is thereafter moved into the horizontal position, initiating a second phase in which the drum is rotated to effect mixing of its contents, initially with said valve means positioned to divert heated air from the drum and thereafter with the valve means positioned to direct heated air to thedrum and initiating a third phase in which the drum is re-inclined to effect discharge of the contents of the drum. r

12. Apparatus as claimed in claim 11, including means for causing a current of cooler air to pass through the drum.

13. Apparatus as claimed in claim 11, wherein a first duct extends from the heater and has an outlet positioned to align with a mouth of the drum when the latter is arranged with its axis horizontal and an exhaust duct opens into the first duct, a damper arrangement being provided to direct the current of air leaving the burner along the first duct to the drum or to direct it into the exhaust duct.

14. Apparatus as claimed in claim 13, wherein an intake duct opens into the first duct, and the damper arrangement is operable to divert air from the burner to the exhaust duct and cool air from the intake duct to the drum.

15. Apparatus as claimed in claim 11, including a hopper for aggregate material, the hopper being arranged to discharge a weighed batch of aggregate into the drum through a mouth thereof when the latter is arranged with its axis inclined to the horizontal, and means for spraying water onto the aggregate.

16. Apparatus as claimed in claim 11, including a nozzle connected to a source of bituminous binder, and disposed so as to direct the binder into the drum through a mouth thereof when the latter is arranged with its axis inclined to the horizontal 17. Apparatus as claimed in claim 11, wherein the drum has a charge mouth at one end and a discharge mouth at the other end.

18. Apparatus as claimed in claim 11, wherein the drum has a common charge and discharge mouth.

19. Apparatus as claimed in claim 17, wherein the drum is provided internally with discharge scrolls in the vicinity of the discharge mouth, said scrolls being effective to cause discharge of the contents of the drum upon rotation of the latter in a reverse direction when 10 the drum is arranged with its axis inclined to the horizontal.

20. Apparatus for the production of coated roadstone, comprising a mixer drum which has two mouths and is supported for rotation about an axis passing through the mouths and for pivotal movement between a position in which the axis of rotation is .generally horizontal and at least one position in which the axis of rotation is inclined to the horizontal, a heater remote from the drum, a first duct extending from the heater, said duct having an outlet positioned to align with one drum mouth when the drum is arranged with its axis horizontal, an exhaust duct opening into the first duct, damper means to direct the current of air passing along the first duct to the drum or to direct it into the exhaust duct, a hopper for aggregate material, the hopper being arranged to discharge a weighed batch of aggregate into the drum through the other mouth when the drum is arranged with its axis inclined to the horizontal, means for spraying water onto the aggregate, and

means for adding bituminous binder to the aggregate. 

1. Apparatus for producing coated roadstone, comprising: a hopper for moist aggregate material, an outlet for a bituminous binder, a mixer, an air heater, a first air supply duct positioned to communicate with the mixer in a mixing position, first valve means for directing along or diverting from said duct a current of air from said heater, an air moving device, a second air supply duct positioned to communicate with the mixer in a discharge position, second valve means for directing along or diverting from said second duct a current of air from said air moving device, and control means for: initiating a first phase in which the mixer is positioned in a charging position to receive aggregate material and binder discharged from the hopper and binder outlet, and thereafter moving the mixer to a mIxing position, initiating a second phase in which the mixer is operated to effect mixing of its contents, first with said first valve means positioned to divert heated air from the first duct and secondly with said first valve means positioned to direct heated air to the first duct, and initiating a third phase in which the mixer is moved to a discharge position and, after the contents of the mixer have at least partly discharged, the second valve means is positioned to direct air from the air moving device to the second air supply duct.
 2. Apparatus as claimed in claim 1, comprising a device for testing the moisture content of the aggregate before it has been fed to the hopper and a device for spraying water onto the aggregate.
 3. Apparatus as claimed in claim 1, wherein the mixer comprises a rotary mixing drum which is pivotably movable about a horizontal axis between the charging, mixing and discharge positions.
 4. Apparatus as claimed in claim 1, wherein the charging position is the same as the discharging position.
 5. Apparatus as claimed in claim 1, wherein the mixer has a first or charging opening and a second or discharging opening.
 6. Apparatus as claimed in claim 5, wherein said first air supply duct is alignable with the second opening in the mixer.
 7. Apparatus as claimed in claim 5, wherein said second air supply duct is alignable with the first opening in the mixer.
 8. Apparatus as claimed in claim 7, wherein the hopper discharges into the second air supply duct.
 9. Apparatus as claimed in claim 5, wherein a third duct is aligned with the first opening in the mixer when the mixer is in its mixing position, and said third duct is the intake duct of the air moving device.
 10. Apparatus as claimed in claim 1, wherein said control system is operative to bring about a reduction in the heat output of the air heater at or about the same time as the first valve means diverts air from the first duct.
 11. Apparatus for the production of coated roadstone, comprising: a mixer drum which has at least one mouth and is supported for rotation about an axis passing through said at least one mouth, said drum being pivotally movable between a position in which the axis of rotation is horizontal and at least one position in which the axis of rotation is inclined to the horizontal, means for charging a moist aggregate and a bituminous binder into the drum when said drum is inclined, heating means for providing a current of hot air, a ducting system for causing the hot air to flow through the drum when said drum is positioned with its axis horizontal, valve means for controlling the flow of hot air through said ducting system, and control means for initiating a first phase in which charging of the drum takes place with the drum in the inclined position and the drum is thereafter moved into the horizontal position, initiating a second phase in which the drum is rotated to effect mixing of its contents, initially with said valve means positioned to divert heated air from the drum and thereafter with the valve means positioned to direct heated air to the drum and initiating a third phase in which the drum is re-inclined to effect discharge of the contents of the drum.
 12. Apparatus as claimed in claim 11, including means for causing a current of cooler air to pass through the drum.
 13. Apparatus as claimed in claim 11, wherein a first duct extends from the heater and has an outlet positioned to align with a mouth of the drum when the latter is arranged with its axis horizontal and an exhaust duct opens into the first duct, a damper arrangement being provided to direct the current of air leaving the burner along the first duct to the drum or to direct it into the exhaust duct.
 14. Apparatus as claimed in claim 13, wherein an intake duct opens into the first duct, and the damper arrangement is operable to divert air from the burner to the exhaust duct and cool air from the intake ducT to the drum.
 15. Apparatus as claimed in claim 11, including a hopper for aggregate material, the hopper being arranged to discharge a weighed batch of aggregate into the drum through a mouth thereof when the latter is arranged with its axis inclined to the horizontal, and means for spraying water onto the aggregate.
 16. Apparatus as claimed in claim 11, including a nozzle connected to a source of bituminous binder, and disposed so as to direct the binder into the drum through a mouth thereof when the latter is arranged with its axis inclined to the horizontal.
 17. Apparatus as claimed in claim 11, wherein the drum has a charge mouth at one end and a discharge mouth at the other end.
 18. Apparatus as claimed in claim 11, wherein the drum has a common charge and discharge mouth.
 19. Apparatus as claimed in claim 17, wherein the drum is provided internally with discharge scrolls in the vicinity of the discharge mouth, said scrolls being effective to cause discharge of the contents of the drum upon rotation of the latter in a reverse direction when the drum is arranged with its axis inclined to the horizontal.
 20. Apparatus for the production of coated roadstone, comprising a mixer drum which has two mouths and is supported for rotation about an axis passing through the mouths and for pivotal movement between a position in which the axis of rotation is generally horizontal and at least one position in which the axis of rotation is inclined to the horizontal, a heater remote from the drum, a first duct extending from the heater, said duct having an outlet positioned to align with one drum mouth when the drum is arranged with its axis horizontal, an exhaust duct opening into the first duct, damper means to direct the current of air passing along the first duct to the drum or to direct it into the exhaust duct, a hopper for aggregate material, the hopper being arranged to discharge a weighed batch of aggregate into the drum through the other mouth when the drum is arranged with its axis inclined to the horizontal, means for spraying water onto the aggregate, and means for adding bituminous binder to the aggregate. 